1. Field of the Invention
The present invention relates to an acceleration sensing device and, more particularly, to an acceleration sensing device for use in an automotive antilock brake system, an air bag system and the like.
2. Description of the Related Art
FIG. 4 is a cross sectional view showing a structure of a known acceleration sensing device. The known acceleration sensing device is a strain gauge type acceleration sensing device making use of the piezo resistance effect. As illustrated in FIG. 4, a base 5 composed of silicon is disposed on a package substrate 7a which is made of conductive material such as Kovar. Also, an acceleration sensing beam 3 composed of silicon is disposed on the base 5 by being fixed thereto at one end of the sensing beam as a cantilever. A weight 6 is fixed to the free end of the acceleration sensing beam 3 and, further, a bridge circuit 1A (See FIG. 5) composed of four gauge resistors 1 is disposed at a position on the acceleration sensing beam 3. The bridge circuit serves as sensor means for detecting the strain of the acceleration sensing beam 3 resulting from the swinging of the weight 6 caused by the acceleration of a body such as an automobile or the like on which the acceleration sensing device is mounted. Operation of the bridge circuit 1A will be described subsequently. The acceleration sensing beam 3 has a thinner portion on which the bridge circuit 1A is disposed and which serves as a diaphragm 4, so that the sensitivity of the bridge circuit 1A for sensing strain is improved. Although the diaphragm 4 may be formed by locally thinning the acceleration sensing beam 3, a diaphragm may be formed separately and joined to the acceleration sensing beam 3.
Further, a connection 2a of the gauge resistors 1 and a wiring conductor 2b are disposed on the acceleration sensing beam 3. The connection 2a and the wiring conductor 2b constitute a wiring means 2. A lead pin 8 extends through the package substrate 7a. The lead pin 8 serves as an output means for outputting electrical signal. The lead pin 8 is electrically insulated from the package substrate 7a by an insulating member such as a glass member 20 and insulating resin. Further, a lead pin 9 is formed integrally with the package substrate 7a so the package substrate 7a is a ground terminal for the bridge circuit 1A. The lead pins 8 and 9 and the wiring means 2 are connected to each other through wires 10 so that an electrical signal indicating a strain detected by the bridge circuit 1A is transmitted to the lead pins 8 and 9 through the wiring means 2 and the wires 10. Further, a cover 7b composed of a metal such as iron or the like is fixedly attached to the package substrate 7a in order to protect the base 5, acceleration sensing beam 3, weight 6 and so on. The package substrate 7a and cover 7b constitute an outer package or an armor 7.
FIG. 5 is a circuit diagram showing an arrangement of the acceleration sensing device of FIG. 4. The bridge circuit 1A comprises of the four gauge resistors 1 as mentioned above. The positive output of the bridge circuit 1A is connected to the positive input of a differential amplifier 11 which is disposed outside of the outer package 7. Also, the negative output of the bridge circuit 1A is connected to the negative input. Operation will be briefly described as follows. When the weight 6 fixed to the acceleration sensing beam 3 moves in response to acceleration of the body which it is attached such as the automobile or the like, a strain is caused in the diaphragm 4. When the gauge resistors 1 receive the strain, their resistance value changes due to the piezo resistance effect and the balance of the bridge circuit 1A is destroyed. When a voltage is applied to the bridge circuit 1A at that time, the amount of imbalance can be determined by detecting the difference in voltage. Therefore, acceleration can be measured from a signal output from the differential amplifier 11 into which the positive output and the negative output from the bridge circuit 1A is input.
The known acceleration sensing device arranged as described above has a problem in that although the base 5 sometimes happens to separate from the package substrate 7a due to vibration of the body to which it is attached such as an automobile, as shown in FIG. 6, if the wires 10 are not severed at that the time, a voltage signal from the bridge circuit 1A is input to the differential amplifier 11 through the lead pin 8 and a signal is output from the differential amplifier 11 based on the voltage signal. Thus, acceleration is erroneously measured without anyone knowing that an abnormal state exists with the base 5 separated from the package substrate 7a.